Free energies and entropies of transfer of hydrobromic and hydroiodic acids from water to t-butyl alcohol + water mixtures from electromotive force measurements at different temperatures (5—35°C)

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Abstract
Free energies, ΔG0 t and entropies, ΔS0 t of transfer of hydrobromic and hydroiodic acids from water to t-butyl alcohol + water mixtures containing 10, 20, 30 and 50 % t-butyl alchol by weight have been evaluated using the corresponding values of the standard potentials, E0, of silver–silver bromide and silver–silver iodide electrodes in these solvents obtained from electromotive force measurements at seven different temperatures ranging from 5 to 35°C performed on the cell Pt, H2(g, 1 atm)|HX(m), solvent|Ag-AgX, where X = Br or I. These and the corresponding values for HCl obtained from E0 data given in literature were used to evaluate ΔG0 t for individual ions by the “extrapolation procedure”. In contrast with other aqueous organic solvents, humps or kinks in the ΔG0 t against composition profiles occur at water-rich compositions reflecting the strong structural characteristics of this solvent system in that region. The variation of entropies of transfer with composition has been interpreted in terms of marked structure-promotion of water and consequent ion-dehydration at small concentrations of t-butyl alcohol and of the formation of alcohol–water complexes at somewhat higher contents of t-butyl alcohol. The irregular order Br– > I– > Cl– in water-rich regions for the entropy profiles is suggested to be due to the formation of charge-transfer complexes of the iodide ion with t-butyl alcohol.

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